1. Field of the Invention
The present invention relates to an image recording medium in which image information can be recorded as an electrostatic latent image.
2. Description of the Related Art
Conventionally, a method has been known which uses, as an image recording medium having a storage unit for storing the amount of charge as latent image charge in accordance with an irradiated electromagnetic wave for recording. For example, in medial radiation photography, a radiation image recording medium (electrostatic recorder) having a photoconductor such as a selenium plate that is sensitive to radioactive rays such as X rays, is used as a photoreceptor. Then, radiation image information is recorded as an electrostatic image by irradiating the radiation image recording medium with X rays and storing the amount of charge in a storage unit in the radiation image recording medium in accordance with a dose of the radiated X rays. Concurrently, the radiation image information is read out from the radiation image recording medium by scanning the radiation image recording medium in which the radiation image information has been recorded by a laser beam or a line light (e.g., U.S. Pat. No. 4,535,468 etc.). By utilizing the radiation image recording medium, it is possible to reduce a dosage of radiation exposure for a subject, as well as improve diagnostic performance etc.
A radiation image recording medium which is capable of high-speed reading response and efficient signal charge taking-out simultaneously, a recording method and a recording device for recording radiation image information on the radiation image recording medium, and a reading method and a reading device for reading out the radiation image information from the radiation image recording medium, have been disclosed in U.S. Pat. No. 6,268,614, U.S. Pat. No. 6,376,857 etc.
In the U.S. Pat. No. 6,268,614 etc., a method and a device for radiation image recording/reading are described, which use a radiation image recording medium constituted by laminating: a first electrode layer for transmitting radioactive rays for recording or a light emitted by the excitation of the radioactive rays; a recording photoconductive layer that exhibits conductivity by being irradiated with the radioactive rays or the light; a charge transportation layer operating as a substantial insulator for latent image charge and as a substantial conductor for transport charge of a polarity reverse to that of the latent image charge; a reading photoconductive layer that exhibits conductivity by being irradiated with an electromagnetic wave for reading; and a second electrode layer for transmitting the reading electromagnetic wave, in this sequential order. The method and the device for radiation image recording/reading also irradiate the first electrode layer of the radiation image recording medium with radioactive rays for recording, record radiation image information as an electrostatic latent image by storing the amount of charge according to a dose of the radiated radioactive rays, in a storage unit formed in a substantial interface between the recording photoconductive layer and the charge transportation layer, and obtain the radiation image information by reading the recorded electrostatic latent image by irradiation with the reading electromagnetic wave.
Further, there has also been proposed a radiation image recording medium where the second electrode layer is a stripe electrode constituted by arraying a number of linear electrodes for transmitting the reading electromagnetic wave in a stripe shape. In this radiation image recording medium, since the latent image charge can be concentrated and stored in the storage unit in accordance with each linear electrode of the stripe electrode, image sharpness can be improved.
In the aforementioned radiation image recording medium, DC voltage is applied so that the first electrode layer can be set to a negative potential and the second electrode layer can be set to a positive potential. Radioactive rays transmitted through an object are irradiated to the first electrode layer. The irradiation of the radioactive rays that have been transmitted through the first electrode layer generates charge pairs in the recording photoconductive layer in accordance with a dose of the radioactive rays. Negative charges are stored as latent image charges in the storage unit, and a radiation image is recorded as an electrostatic image.
When the reading electromagnetic wave is irradiated to the second electrode layer of the radiation image recording medium, this electromagnetic wave is transmitted through the second electrode layer to irradiate the reading photoconductive layer. As a result, charge pairs are generated in the reading photoconductive layer. Positive charges of the charge pairs are passed through the charge transportation layer to be coupled with the negative charges stored in the storage unit, then the negative charges are coupled again with the positive charges applied to the second electrode layer, whereby generating electrical discharge. This discharging causes a voltage change between the first electrode layer and the second electrode layer. Then, an electrostatic image is read by detecting the voltage change as a current change with a current detection amplifier or the like.
The reading photoconductive layer in the radiation image recording medium is made of a-Se (amorphous selenium) in most cases because of advantages of high dark resistance and a high reading response speed. However, in a selenium film in an amorphous state, interfacial crystallization progresses during a deposition process of film formation, at interfaces with other materials to increase charge injection from the electrode, consequently causing a problem of S/N reduction. If a transparent oxide film, particularly ITO, is used as an electrode material, interfacial crystallization conspicuously progresses in an interface between the electrode material and a-Se.
Thus, to prevent the problem of the interfacial crystallization in the reading photoconductive layer, there has been proposed a provision of a suppression layer made of an organic polymer for suppressing interfacial crystallization between the electrode layer irradiated with a reading light and the reading photoconductive layer.
However, if the suppression layer is formed between the electrode layer irradiated with the reading electromagnetic wave and the reading photoconductive layer, there is a drawback that interference occurs with coupling between negative charge generated in the reading photoconductive layer during reading and positive charge in the electrode irradiated with the reading electromagnetic wave, i.e., a reduction occurs in photoinduction discharging efficiency in the reading photoconductive layer to lower reading efficiency. This reading efficiency reduction is observed conspicuously in a region where irradiation intensity of a recording electromagnetic wave is weak, i.e., a region where photoinduction discharging must be carried out under a low electric field.